Exhaust Gas Purifying Catalyst

ABSTRACT

This exhaust gas purifying catalyst is provided with a substrate and a catalyst layer formed on a surface of the substrate. The catalyst layer contains zeolite particles that support a metal, and a rare earth element-containing compound that contains a rare earth element. The rare earth element-containing compound is added in such an amount that the molar ratio of the rare earth element relative to Si contained in the zeolite is 0.001 to 0.014 in terms of oxides.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 16/485,513 filed Aug. 13, 2019, which is a national stage ofInternational Application No. PCT/JP2018/005691, filed Feb. 19, 2018,which claims priority to Japanese Application No. 2017-029297, filedFeb. 20, 2017 and Japanese Application No. 2017-224344, filed Nov. 22,2017, the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an exhaust gas purifying catalyst. Morespecifically, the present invention relates to an exhaust gas purifyingcatalyst that cleans exhaust gases emitted from an internal combustionengine such as a motor vehicle engine.

BACKGROUND ART

Exhaust gases emitted from internal combustion engines such as motorvehicle engines contain harmful components such as carbon monoxide (CO),hydrocarbons (HC) and nitrogen oxides (NOx). These harmful componentsare known to be primary causes of atmospheric pollution. Three-waycatalysts obtained by supporting noble metals such as platinum, rhodiumand palladium on porous carriers are widely used as catalysts forpurifying a broad range of harmful components in exhaust gases. Amongthese harmful components, however, NOx are difficult to clean withthree-way catalysts. As a result, SCR catalysts (NOx selective catalyticreduction catalysts) have been developed as useful catalysts able toclean NOx. Patent Literature 1 and 2 are cited as technical documentsrelating to SCR catalysts.

In a typical configuration of a SCR catalyst, a catalyst layer thatcontains a SCR catalyst is provided on a surface of a substrate such asa honeycomb or filter substrate. Metal-supporting zeolites such ascopper-supporting zeolites and iron-supporting zeolites are known asexamples of SCR catalysts. By supplying a reducing agent (for example,urea water) to a filter provided with this type of catalyst layer, thereducing agent is hydrolyzed and generates ammonia. If the ammonia isadsorbed on a SCR catalyst, NOx in exhaust gases are cleaned by thereducing action of the ammonia (for example, 4NH₃+2NO₂+2NO→4N₂+6H₂O).

CITATION LIST Patent Literature

Patent Literature 1: Japanese Translation of PCT Application No.2013-537846 Patent Literature 2: Japanese Patent Application PublicationNo. H08-57324

SUMMARY OF INVENTION

However, according to findings by the inventors of the presentinvention, if a SCR catalyst obtained using a metal-supporting zeolitementioned above is exposed to water vapor in a high temperatureenvironment, degradation of the zeolite occurs. This leads to theproblem of catalyst performance (for example, NOx purifying performance)decreasing. Therefore, an exhaust gas purifying catalyst in whichcatalyst performance is unlikely to decrease even when exposed to watervapor in a high temperature environment, in other words, an exhaust gaspurifying catalyst having excellent hydrothermal durability, isrequired.

With these circumstances in mind, the primary objective of the presentinvention is to provide an exhaust gas purifying catalyst havingexcellent hydrothermal durability.

The inventors of the present invention found that by adding a rare earthelement-containing compound to a catalyst layer containing ametal-supporting zeolite, hydrothermal durability of an exhaust gaspurifying catalyst was improved. In addition, the inventors of thepresent invention found that by adjusting the molar ratio of a rareearth element component contained in the rare earth element-containingcompound and a silicon (Si) component contained in the zeolite to anappropriate molar ratio, it was possible to effectively improvehydrothermal durability without causing a decrease in NOx purifyingperformance of the catalyst as a whole, and thereby completed thepresent invention.

The present invention provides an exhaust gas purifying catalyst whichis disposed in an exhaust pathway of an internal combustion engine andcleans exhaust gas emitted from the internal combustion engine. Thisexhaust gas purifying catalyst is provided with a substrate and acatalyst layer formed on a surface of the substrate. The catalyst layercontains zeolite particles that support a metal, and a rare earthelement-containing compound that contains a rare earth element. Anamount of the rare earth element-containing compound contained is suchan amount that a molar ratio of the rare earth element relative to Sicontained in the zeolite particles is 0.001 to 0.014 in terms of oxides.By adding the rare earth element-containing compound to the zeoliteparticles so as to attain a specific molar ratio in this way, it ispossible to effectively improve hydrothermal durability without causinga decrease in the NOx purifying performance of the catalyst as a whole.Therefore, it is possible to achieve high NOx purifying performance andadvantageously maintain this purifying performance over a long period oftime.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the rare earth element-containing compound is adsorbed on thesurface of the zeolite particles. By disposing the rare earthelement-containing compound on the surface of the zeolite particles, theadvantageous effect of improving hydrothermal durability can be betterexhibited.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the relationship between an average particle diameter D1 of thezeolite particles and an average particle diameter D2 of the rare earthelement-containing compound satisfies the following formula:0.005<(D2/D1)<0.5. If the zeolite particles and the rare earthelement-containing compound satisfy a specific average particle diameterratio, the advantageous effect of improving hydrothermal durability canbe better exhibited.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the average particle diameter D2 of the rare earthelement-containing compound is 100 nm or less. In this way, thehydrophilic properties of the surface of the zeolite particles can beefficiently lowered and higher hydrothermal durability can be exhibited.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, when an amount of the rare earth element at a cross section of azeolite particle is measured using an Electron Probe Micro Analyzer(EPMA), the amount of the rare earth element present at the surface ofthe zeolite particle is greater than the amount of the rare earthelement present in the inner part of the zeolite particle. In this way,an exhaust gas purifying catalyst having high hydrothermal durabilityand excellent catalytic activity can be advantageously realized.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the rare earth element-containing compound contains at least oneof lanthanum oxide and lanthanum hydroxide. The rare earthelement-containing compound can effectively contribute to an improvementin hydrothermal durability.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the metal supported on the zeolite particles is Cu or Fe. Themetal can effectively contribute to an improvement in catalyticactivity.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the molar ratio of Si and Al in terms of oxides (SiO₂/Al₂O₃) inthe zeolite particles is 5 to 20. The zeolite can effectively contributeto an improvement in NOx purifying performance.

In a preferred aspect of the exhaust gas purifying catalyst disclosedhere, the zeolite particles contain at least one type of zeoliteselected from among those represented by structure codes CHA, AFX, AEI,LTA and BEA defined by the International Zeolite Association (IZA). Thezeolite can effectively contribute to an improvement in NOx purifyingperformance

In addition, the present invention provides an exhaust gas purifyingapparatus provided with the exhaust gas purifying catalyst and areducing agent supply mechanism which supplies a reducing agent forgeneration of ammonia to the exhaust gas at a position upstream in theexhaust pathway as compared to a position of the exhaust gas purifyingcatalyst. An exhaust gas purifying apparatus that exhibits higherhydrothermal durability and better NOx purifying performance than in thepast is realized by this configuration.

In addition, the present invention provides a catalyst body used in theexhaust gas purifying catalyst. This catalyst body contains zeoliteparticles that support a metal, and a rare earth element-containingcompound that contains a rare earth element, and an amount of the rareearth element-containing compound contained is such an amount that amolar ratio of the rare earth element relative to Si contained in thezeolite particles is 0.001 to 0.014 in terms of oxides.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram that illustrates an exhaust gaspurifying catalyst according to one embodiment.

FIG. 2 is a diagram that schematically illustrates a rib wall portion inan exhaust gas purifying catalyst according to one embodiment.

FIG. 3 is a diagram that schematically illustrates a substrate andcatalyst layer formed on a surface of the substrate in one embodiment.

FIG. 4 is a secondary electron image of zeolite particles in Example 2.

FIG. 5 is a Si element mapping image of zeolite particles in Example 2.

FIG. 6 is an Al element mapping image of zeolite particles in Example 2.

FIG. 7 is a La element mapping image of zeolite particles in Example 2.

FIG. 8 is a graph that shows FT-IR results for zeolite particles inExample 2 and Example 14.

FIG. 9 is a graph in which NOx purifying rates are compared for Examples1 to 6 and 14.

FIG. 10 is a graph in which NOx purifying rates are compared forExamples 7 to 11 and 15.

FIG. 11 is a graph in which NOx purifying rates are compared forExamples 18 to 22.

FIG. 12 is a graph in which NOx purifying rates are compared forExamples 23 to 27.

FIG. 13 is a graph in which NOx purifying rates are compared forExamples 28 to 31.

DESCRIPTION OF EMBODIMENTS

Based on the drawings, explanations will now be given of preferredembodiments of the present invention. Moreover, matters which areessential for carrying out the invention (for example, ordinary matterssuch as those relating to the arrangement of the exhaust gas purifyingcatalyst) and which are matters other than those explicitly mentioned inthe present specification (for example, the composition of the catalystlayer, and the like) are matters that a person skilled in the art couldunderstand to be matters of design on the basis of the prior art in thistechnical field. The present invention can be carried out on the basisof the matters disclosed in the present specification and common generaltechnical knowledge in this technical field. Moreover, cases wherenumerical ranges in the present specification are written as A to B(here, A and B are arbitrary numbers) mean not less than A and not morethan B.

<Exhaust Gas Purifying Catalyst>

The exhaust gas purifying catalyst disclosed here comprises a substrateand a catalyst layer formed on a surface of the substrate.

FIG. 1 is a schematic diagram of an exhaust gas purifying catalyst 100.The exhaust gas purifying catalyst 100 is disposed in an exhaust pathwayof an internal combustion engine. The exhaust gas purifying catalyst 100has a mechanism that cleans exhaust gases emitted from an internalcombustion engine. For example, internal combustion engines comprisemainly gasoline engines and diesel engines. The exhaust gas purifyingcatalyst 100 is provided with a substrate 10. The substrate 10 has aplurality of regularly arranged cells 12 and rib walls 14 that configurethe cells 12.

<Substrate>

For the substrate 10, it is possible to use a variety of conventionalmaterials and forms that were used in the past in such applications. Forexample, a substrate formed from a ceramic such as cordierite or siliconcarbide (SiC) or an alloy such as stainless steel can be advantageouslyused. In the present embodiment, the substrate 10 is a honeycombsubstrate having a honeycomb structure. The substrate 10 is formed intoan overall cylindrical shape that extends in the exhaust gas flowdirection (shown by the arrows in FIGS. 1 and 2). The substrate 10 issuch that the external shape of the substrate as a whole is cylindrical.However, the form of the substrate 10 is not particularly limited, andmay be, for example, a foam-like form or pellet-like form in addition toa honeycomb form. In addition, the external shape of the substrate as awhole may be, for example, an elliptic cylinder or a polygonal cylinderinstead of a circular cylinder.

A straight flow type substrate, such as a honeycomb substrate, can begiven as an example of the substrate 10. A straight flow type substrateis such that the external shape of the substrate as a whole iscylindrical, and a plurality of through holes (cells) are provided asexhaust gas pathways in the cylindrical axis direction of the substrate.In addition, exhaust gases can come into contact with partitions (ribwalls) that divide the cells. A wall flow type substrate, such as afilter substrate, can be given as another example of the substrate 10. Awall flow structure substrate typically has inlet-side cells in whichonly the exhaust gas inlet side end is open, outlet-side cells in whichonly the exhaust gas outlet side end is open, and porous partitionswhich divide the inlet-side cells from the outlet-side cells. In suchcases, an exhaust gas that flows in from an inlet-side cell passesthrough a porous cell partition and is discharged to the outside from anoutlet-side cell. In addition, while the exhaust gas passes through theporous cell partition, particulate matter is trapped in pores inside thecell partition.

FIG. 2 is a diagram that schematically illustrates the configuration ofa surface portion of a rib wall 14 in the substrate 10. As shown in FIG.2, the rib wall 14 is provided with a catalyst layer 20 formed on asurface of the substrate 10. FIG. 3 is a diagram that schematicallyillustrates the substrate 10 and the catalyst layer 20. As shown in FIG.3, the catalyst layer 20 contains zeolite particles 22 that support ametal, and a rare earth element-containing compound 24.

<Zeolite>

The zeolite particles 22 contain at least Si as an element thatconstitutes the basic skeleton. The zeolite particles typically furthercontain aluminum (Al). The zeolite particles 22 are, for example, aporous crystalline aluminosilicate. The zeolite particles 22 may be, forexample, particles in which a cation such as Al or phosphorus (P) iscontained within the skeleton of a tetrahedral SiO₄ structure in thezeolite. In addition, the zeolite particles 22 may be, for example,particles having a bonded element portion such as Si—O—Al or P—O—Al inthe basic skeleton. β zeolites and silico-alumino-phosphate (SAPO)zeolites can be given as specific examples of the zeolite particles 22.

Skeletal structures of zeolites have been listed in a database by theInternational Zeolite Association (IZA), and skeletal structures havebeen given structure codes comprising three uppercase letters. Astructure code shows the geometric structure of a skeleton. Zeolitesrepresented by the structure codes AEI, AFX, AFT, ATT, BEA, CHA, DDR,IFY, JST, KFI, LEV, LOV, LTA, OWE, PAU, RHO, RSN, SAV, SFW, TSC, UEI,UFI and VSV can be given as preferred examples of the zeolite particles22. One or two or more of these can be advantageously used. Of these,CHA, AFX, AEI, LTA and BEA are preferred, and chabazite (CHA) typezeolites in which the average pore diameter is approximately the same asthe size of a molecule of NO or NO₂ (approximately 0.38 nm) areparticularly preferred.

The compositional ratio of a Si component and an Al component in thezeolite particles 22 is not particularly limited, but should generallybe such that the SiO₂/Al₂O₃ molar ratio in terms of oxides is 1 to 400,preferably 2 to 200, more preferably 3 to 100, further preferably 5 to50, and particularly preferably 5 to 20, and is 7 to 10 in one example,for example less than 10. In this way, the hydrothermal durability ofthe exhaust gas purifying catalyst 100 can be more effectively improvedand significantly higher NOx purifying performance can be achieved.

The metal is supported on the zeolite particles 22. In this way, NOx inexhaust gases can be efficiently cleaned. If Al³⁺ is introduced at aSi⁴⁺ position in a tetrahedral SiO₄ structure in a zeolite particle 22,one ion exchange site is formed. In one example, an arbitrary metalcation can be supported by means of ion exchange in the zeolite particle22 using this ion exchange site. That is, the zeolite particles 22 maybe an ion exchanged zeolite. Examples of metals that are supported bymeans of ion exchange in the zeolite particles 22 typically includemetals other than Al, for example, transition metals such as copper(Cu), iron (Fe) and vanadium (V). The amount of metal supported is notparticularly limited, but is generally 0.5 to 10 mass %, and shouldtypically be 1 to 6 mass %, for example 1 to 5 mass %, if the overallmass of metal-supporting zeolite particles 22 is taken to be 100 mass %.

Zeolite particles 22 may contain optional metal components in additionto the Si, Al, P, Cu, Fe and V mentioned above. Examples of suchoptional metal components include alkali metal elements such as sodium(Na) and potassium (K); alkaline earth metal elements such as magnesium(Mg) and calcium (Ca); cobalt (Co), nickel (Ni), zinc (Zn), silver (Ag),lead (Pb), vanadium (V), chromium (Cr), molybdenum (Mo), yttrium (Y),cerium (Ce), neodymium (Nd), tungsten (W), indium (In), iridium (Ir) andtitanium (Ti).

An average particle diameter D1 of the zeolite particles 22 is notparticularly limited, but is generally 0.1 μm or more, preferably 0.2 μmor more, more preferably 0.3 μm or more, and further preferably 0.4 μmor more. The upper limit of the average particle diameter D1 is notparticularly limited, but should generally be 10 μm or less. From aperspective such as uniformly disposing the rare earthelement-containing compound 24 at the surface of the zeolite particles22, the average particle diameter D1 is preferably 8 μm or less, morepreferably 5 μm or less, and further preferably 3 μm or less. Therefore,the average particle diameter D1 of the zeolite particles 22 should be,for example, 0.3 to 3 μm.

Moreover, the average particle diameter D1 of the zeolite particles 22should be determined by measurements obtained on the basis of a laserscattering method or observations using a scanning electron microscope(SEM). For example, the particles are first imaged at a magnification of20,000 times using a field emission-scanning electron microscope(FE-SEM), and a secondary electron image is obtained. Next, the obtainedsecondary electron image is binarized using image processing software(WinROOF (registered trademark)), and particles are detected. Next, thedetected particles are subjected to particle size analysis, and theparticle diameter is calculated on the assumption that the particles arecompletely circular. This is then calculated as the particle diameter ofthe particles. Particle size analysis should be performed on, forexample, 20 to 50 zeolite particles. The number-based average particlediameter D1 can be determined by taking the arithmetic mean of theseresults. This is also the case in the examples given later.

<Rare Earth Element-Containing Compound>

Typically, it is preferable for the rare earth element-containingcompound 24 to contain one or two or more elements selected from amonglanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu),yttrium (Y) and scandium (Sc). Of these, any of La, Ce, Pr and Y arepreferred, La and Ce are more preferred, and La is particularlypreferred.

The type of rare earth element-containing compound is not particularlylimited. The rare earth element-containing compound may be, for example,an oxide, a hydroxide, a nitride, a carbide, a boride, a sulfide, achloride, a fluoride, a carbonate, a bicarbonate, a sulfate, a nitrateor an oxalate. One or two or more of these can be used. Of these,oxides, hydroxides, carbides and borides are preferred, and oxides andhydroxides are particularly preferred. Specific examples of the rareearth element-containing compound include lanthanum oxide (La₂O₃),cerium oxide (CeO₂), praseodymium oxide (Pr₆O₁₁), yttrium oxide (Y₂O₃),lanthanum hydroxide (La(OH)₃), cerium hydroxide (Ce(OH)₃), praseodymiumhydroxide and yttrium hydroxide (Y(OH)₃).

In the present embodiment, the rare earth element-containing compound 24is used by being added to the zeolite particles 22. In this way, thehydrothermal durability of the exhaust gas purifying catalyst 100 can bemore effectively improved. The reason why such an effect can be achievedcannot be explained particularly definitively, but is thought to be asfollows. The zeolite particles 22, which contain Si as an element thatconstitutes the basic skeleton, include many silanol groups (Si—OH) atterminals of the basic skeleton and at edges of surface defects and thelike. Because silanol groups are hydrophilic, moisture is adsorbed atthe surface of zeolite particles 22. If zeolite particles 22 havingmoisture adsorbed on the surface thereof are exposed to hightemperatures, the moisture attacks the zeolite skeleton. In addition,bonded element portions such as Si—O—Al that constitute the skeleton ofthe zeolite particles 22 are decomposed. It is thought that the skeletalstructure of the zeolite disintegrates as a result. It is thought thatthis skeletal structure disintegration is a primary cause of a reductionin catalytic activity.

However, in the exhaust gas purifying catalyst 100, in which the rareearth element-containing compound 24 is added to the zeolite particles22, a silanol group at the surface of a zeolite particle 22 reacts withthe rare earth element-containing compound 24. In this way, theproportion of silanol groups present at the surface of the zeoliteparticles 22 is lower than in the past. As a result, the hydrophilicityof the surface of the zeolite particles 22 decreases. Therefore, thestructure of the zeolite skeleton is unlikely to disintegrate even ifexposed to water vapor in a high temperature environment. It is thoughtthat this contributes to an improvement in hydrothermal durability.

In the present embodiment, the content (added quantity) of the rareearth element-containing compound 24 is such an amount that the molarratio of the rare earth element relative to Si contained in the zeoliteparticles 22 is 0.001 or more in terms of oxides. That is, an amountsuch that (rare earth element oxide/SiO₂)≥0.001. If this added quantityfalls within such a range, the proportion of silanol groups at thesurface of the zeolite particles 22 is effectively reduced and thehydrothermal durability of the exhaust gas purifying catalyst 100 isimproved. From a perspective such as further improving hydrothermaldurability, this molar ratio is preferably 0.0015 or more, morepreferably 0.002 or more, further preferably 0.003 or more, andparticularly preferably 0.004 or more.

In addition, the added quantity of the rare earth element-containingcompound 24 is such an amount that the molar ratio of the rare earthelement relative to Si contained in the zeolite particles 22 is 0.014 orless in terms of oxides. That is, an amount such that (rare earthelement oxide/SiO₂)≤0.014. If the added quantity falls within such arange, the surface of the zeolite particles 22 is not excessivelycovered by the rare earth element-containing compound 24. As a result,it is possible to suppress a reduction in gas permeability and ensurestable contact with exhaust gases. Therefore, high NOx purifyingperformance can be achieved. From a perspective such as bettersuppressing a decrease in gas permeability, the molar ratio mentionedabove is preferably 0.012 or less, more preferably 0.01 or less, furtherpreferably 0.008 or less, and particularly preferably 0.006 or less.

The feature disclosed here can be advantageously implemented by anembodiment in which the molar ratio of rare earth element relative to Sicontained in the zeolite particles 22 is generally 0.003 to 0.01, andespecially 0.0035 to 0.009, for example 0.004 to 0.008. Moreover, therare earth element content can be a value obtained in terms of oxideusing La₂O₃, CeO₂, Pr₆O₁₁, Nd₂O₃, Sm₂O₃, Eu₂O₃, Gd₂O₃, Th₂O₃, Dy₂O₃,Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, Y₂O₃ or Sc₂O₃.

In a preferred aspect, the rare earth element-containing compound 24 islocalized at the surface of the zeolite particles 22. If the rare earthelement-containing compound 24 is localized at the surface of thezeolite particles 22, the hydrophilicity of the surface of the zeoliteparticles 22 is efficiently lowered. In addition, in a preferred aspect,the rare earth element-containing compound 24 adheres to the surface ofthe zeolite particles 22. In other words, the rare earthelement-containing compound 24 is integrated with the zeolite particles22 by means of physical and/or chemical bonding. In this way, theadvantageous effect of improving hydrothermal durability is betterexhibited.

The rare earth element-containing compound 24 is typically in the formof particles. An average particle diameter D2 of the rare earthelement-containing compound 24 is not particularly limited, but isgenerally nanometer-sized, and should be, for example, 100 nm or less.In this way, the rare earth element-containing compound 24 can be evenlydisposed on the surface of the zeolite particles 22, and the proportionof silanol groups present at the surface of the zeolite particles 22 canbe more effectively reduced. From a perspective such as uniformlydisposing the rare earth element-containing compound 24 at the surfaceof the zeolite particles 22, the average particle diameter D2 of therare earth element-containing compound 24 is preferably 90 nm or less,more preferably 75 nm or less, and further preferably 50 nm or less.

The lower limit of the average particle diameter D2 of the rare earthelement-containing compound 24 is not particularly limited, but isgenerally 1 nm or more, preferably 5 nm or more, more preferably 10 nmor more, and further preferably 20 nm or more. If the average particlediameter D2 is too low, the rare earth element-containing compound 24may, in some cases, penetrate into the inner part of the zeoliteparticles 22. If the rare earth element-containing compound 24penetrates into the inner part of the zeolite particles 22, a metalsupported on the zeolite (for example, Cu) may be replaced by the rareearth element, which may lead to loss of a metal that is an activespecies. As a result, the purifying performance (for example, NOxpurifying performance) of the catalyst as a whole may deteriorate.Therefore, the average particle diameter D2 of the rare earthelement-containing compound 24 should be, for example, 25 to 100 nm.

Moreover, the average particle diameter D2 of the rare earthelement-containing compound should be determined by measurementsobtained on the basis of a dynamic light scattering method orobservations using a scanning electron microscope (SEM) or transmissionelectron microscope (TEM). The measurement method to be used depends onthe average particle diameter D2.

For example, for particles having an average particle diameter of lessthan 0.5 μm, the particle diameter should be measured in a state priorto the particulate rare earth element-containing compound 24 beingadhered to the surface of the zeolite particles 22. Specifically, asample is first prepared by diluting a sol, in which the rare earthelement-containing compound 24 to be measured is dispersed (at a rareearth element-containing compound concentration of 10% to 20%), asappropriate with water. Next, the sample is irradiated with laser lightand the particle diameter is measured by detecting scattered light. Inother words, the particle diameter should be measured using a dynamiclight scattering method. These measurements can be carried out using,for example, a “Zetasizer (registered trademark) Nano S” available fromMalvern Panalytical Ltd.

In addition, for particles having an average particle diameter of 0.5 μmor more, the particle diameter should be measured in a state in whichthe particulate rare earth element-containing compound is adhered to thesurface of the zeolite particles 22. Specifically, the particulate rareearth element-containing compound 24 adhered to the surface of thezeolite particles 22 is first imaged at a magnification of 20,000 timesusing a field emission-scanning electron microscope (FE-SEM), and asecondary electron image is obtained. Here, whether or not the particlesbeing measured are the rare earth element-containing compound can bedetermined by energy dispersive X-ray spectroscopy (EDX). In otherwords, the only particles for which a rare earth element is detected bymeans of EDX electron analysis are particles of the rare earthelement-containing compound. Next, the obtained image is binarized usingimage analysis software (WinROOF (registered trademark)) so as to detectparticles. Next, the detected particles are subjected to particle sizeanalysis, and the particle diameter is calculated on the assumption thatthe particles are completely circular. This is then calculated as theparticle diameter of the particles. Particle size analysis should beperformed on, for example, 20 to 50 rare earth element-containingcompound particles. The number-based average particle diameter D2 can bedetermined by taking the arithmetic mean of these results. This is alsothe case in the examples given later.

From the perspective of exhibiting the advantageous effect of adding therare earth element-containing compound 24 to the zeolite particles 22 ata higher level, it is preferable for the average particle diameter D1 ofthe zeolite particles 22 and the average particle diameter D2 of therare earth element-containing compound 24 to satisfy the followingrelationship: 0.005<(D2/D1)<0.5. The feature disclosed here can beadvantageously implemented by an embodiment in which, for example, therelationship between the average particle diameter D1 and the averageparticle diameter D2 is such that 0.008<(D2/D1)<0.4, more preferably0.01<(D2/D1)<0.3, further preferably 0.03<(D2/D1)<0.2, and particularlypreferably 0.05<(D2/D1)<0.15. In addition, the average particle diameterD1 is preferably at least 100 nm larger, and more preferably at least200 nm larger, than the average particle diameter D2. In addition, avalue obtained by subtracting the average particle diameter D2 from theaverage particle diameter D1 (D1-D2) is preferably 1000 nm or less, morepreferably 800 nm or less, further preferably 600 nm or less, andparticularly preferably 500 nm or less.

In a preferred aspect, when an amount of the rare earth element at across section of a zeolite particle 22 is measured using an electronprobe microanalyzer (EPMA), the amount of the rare earth element presentat the surface of the zeolite particle 22 is greater than the amount ofthe rare earth element present in the inner part of the zeolite particle22. In other words, if the amount of the rare earth element present atthe surface of a zeolite particle 22 is denoted by A and the amount ofthe rare earth element present in the inner part of the zeolite particle22 is denoted by B, the relationship A>B should be satisfied.

The amount B of the rare earth element present in the inner part of thezeolite particle 22 should generally be, for example, half or less ofthe amount A of the rare earth element present at the surface of azeolite particle 22. That is, A and B above should satisfy therelationship B/A≤0.5, and are preferably such that B/A≤0.3, morepreferably such that B/A≤0.1, further preferably such that B/A 0.05, andparticularly preferably such that B/A 0.01. The feature disclosed herecan be advantageously implemented by an embodiment in which the amount Bof the rare earth element present in the inner part of the zeoliteparticle 22 is substantially 0 (zero). By constituting in this way,defects are unlikely to occur, such as a metal supported on the zeolite(for example, Cu) being replaced by a rare earth element in the innerpart of a zeolite particle 22. As a result, a decrease in purifyingperformance (for example, NOx purifying performance) of the catalyst asa whole is suppressed and the advantageous effect of improvedperformance through addition of the rare earth element-containingcompound 24 is more advantageously achieved.

A variety of adhesion methods can be used as the method for disposingthe rare earth element-containing compound 24 on the surface of thezeolite particles 22. Examples thereof include the following methods:(1) a wet method comprising mixing a rare earth element-containingcompound sol, a solvent such as water or an alcohol, and the zeoliteparticles 22 so as to cause the rare earth element-containing compound24 to adhere to the surface of the zeolite particles 22; and (2) a drymethod comprising mixing a rare earth element-containing compound soland the zeolite particles 22 in the absence of a solvent so as to causethe rare earth element-containing compound 24 to adhere to the surfaceof the zeolite particles 22. From a perspective such as evenly disposingthe rare earth element-containing compound on the surface of the zeoliteparticles 22, a dry method is preferred.

In a preferred aspect, the average particle diameter of the rare earthelement-containing compound in the rare earth element-containingcompound sol that serves as a raw material is generally 1 to 100 nm. Ifthis average particle diameter falls within such a range, it is possibleto prevent the rare earth element-containing compound 24 frompenetrating into the inner part of the zeolite particles 22. Inaddition, it is possible to cause the rare earth element-containingcompound 24 to adhere to the zeolite particles 22 in a highly dispersedstate. As a result, it is possible to uniformly cover the surface of thezeolite particles 22 with the rare earth element-containing compound 24.

The catalyst layer 20 may, if necessary, contain components other thanthe zeolite particles 22 and the rare earth element-containing compound24. The catalyst layer 20 includes, for example, a carrier and a noblemetal supported on the carrier. The carrier can contain substances usedas this type of carrier in the past, such as alumina (Al₂O₃), zirconia(ZrO₂), and solid solutions and complex oxides thereof. The carrierpreferably contains alumina. For example, metal catalysts such asplatinum (Pt), palladium (Pd), rhodium (Rh) and silver (Ag) and solidsolutions, alloys, and the like, containing these metal catalysts can beadvantageously used as the noble metal supported on the carrier. Thesenoble metals preferably have an oxidation catalyst function, that is, acatalyst function capable of removing excess ammonia present in exhaustgases which has not been used for NOx purifying. Pt is an example of anoble metal having an oxidation catalyst function.

<Method for Forming Catalyst Layer>

In order to form of the catalyst layer 20, a slurry containing thezeolite particles 22 to which the rare earth element-containing compound24 is adhered, an appropriate solvent (for example, water) and othercomponents that constitute catalyst layer should be applied to a surfaceof the substrate 10. At this point, the slurry may contain a binder 26from the perspective of increasing adhesion of the slurry to the surfaceof the substrate 10. It is preferable to use, for example, a silica solor an alumina sol as the binder. The viscosity of the slurry should beadjusted as appropriate so that the slurry can flow easily into cells 12of the substrate 10. The slurry applied to the surface of the substrate10 is dried by heating, and then fired. The solvent is removed in thisway. The drying conditions may depend on the form and dimensions of thesubstrate 10 and carrier, but are generally a temperature of 80° C. to300° C., for example 100° C. to 250° C. The firing conditions aregenerally a temperature of 400° C. to 1000° C., for example 500° C. to700° C.

The coating amount (formed amount) of the catalyst layer 20 is notparticularly limited. For example, in cases where a straight flow typesubstrate (for example, a honeycomb substrate) is used as the substrate,the coating amount per unit volume of catalyst is generallyapproximately 50 to 300 g/L, and typically 70 to 250 g/L, for example150 to 220 g/L. In addition, in cases where a wall flow type substrate(for example, a filter substrate) is used as the substrate, the coatingamount per unit volume of catalyst is generally approximately 20 to 200g/L, and typically 50 to 180 g/L, for example 60 to 150 g/L. Moreover, aunit volume (1 L) of catalyst means the bulk volume that includes notonly the pure volume of the substrate 10, but also the volume of voids(cells) 12 in the inner part of the catalyst, that is, the catalystlayer 20 formed inside voids (cells) 12.

As mentioned above, the exhaust gas purifying catalyst 100 caneffectively improve hydrothermal durability without causing adeterioration in the purifying performance (for example, NOx purifyingperformance) of the catalyst as a whole. Therefore, the exhaust gaspurifying catalyst 100 can be advantageously used as a constituentelement of a variety of exhaust gas purifying apparatuses, for exampleas a SCR catalyst, a three-way catalyst, an NOx storage-reduction (NSR)catalyst or a catalyst obtained by combining these. A specific examplethereof is an exhaust gas purifying apparatus provided with the exhaustgas purifying catalyst 100 and a reducing agent supply mechanism whichsupplies a reducing agent for generation of ammonia at a positionupstream in the exhaust pathway as compared to a position of the exhaustgas purifying catalyst 100.

The reducing agent supply mechanism is configured so as to supply areducing agent to an exhaust gas at a position upstream as compared to aposition of the exhaust gas purifying catalyst 100. The reducing agentsupply mechanism is a reducing agent solution supply mechanism that isconfigured so as to supply a reducing agent solution, for example ureawater, from a position upstream in the direction of flow of the exhaustgas as compared to a position of the exhaust gas purifying catalyst 100.The reducing agent solution supply mechanism typically comprises a spraynozzle, a pump and a tank. The spray nozzle is connected to the tank bya flow pathway. The pump is disposed in the flow pathway between thespray nozzle and the tank, and supplies the reducing agent solution inthe tank to the spray nozzle. The reducing agent solution supplied tothe spray tank is sprayed into the exhaust gas in the exhaust pathway,and is transported to the downstream side of the exhaust pathwaytogether with the exhaust gas. The reducing agent solution is hydrolyzedand generates ammonia. The ammonia is adsorbed on the catalyst layer 20of the exhaust gas purifying catalyst 100. More specifically, theammonia is adsorbed on the zeolite particles (SCR catalyst) on which therare earth element-containing compound 24 is adhered. NOx in the exhaustgas are converted into nitrogen and water by the reducing action of theammonia adsorbed on the catalyst layer 20. In this way, NOx in theexhaust gas are cleaned. According to the features disclosed here, it ispossible to realize an exhaust gas purifying apparatus which has higherhydrothermal durability and better purifying performance (for example,NOx purifying performance) than in the past. For example, it is possibleto realize an exhaust gas purifying apparatus in which catalystperformance is unlikely to deteriorate even exposed to water vapor inhigh temperature environments having temperatures of 500° C. or higher,and even 750° C. or higher.

Explanations will now be given of test examples relating to the presentinvention, but it is not intended that the present invention is limitedto these test examples.

TEST EXAMPLE 1 (1) Preparation of Exhaust Gas Purifying Catalyst Example1

217 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) wasmixed with a solution obtained by mixing 251 g of pure water, 21 g of aSiO₂ sol and 31 g of a La₂O₃ sol, and stirred for 15 minutes. A slurryof La₂O₃-adhered zeolite particles was prepared in this way. The La₂O₃sol was added at a quantity whereby the molar ratio of La contained inthe La₂O₃ relative to Si contained in the zeolite (La₂O₃/SiO₂) was0.00113 in terms of oxides. In addition, the amount of water in theslurry was adjusted so as to attain a slurry viscosity such that theslurry could be coated on a cordierite honeycomb substrate. The obtainedslurry was coated on a cordierite honeycomb substrate at a quantitywhereby the coating amount of the Cu ion exchange zeolite (the mass per1 L of substrate volume) was 180 g/L after firing. Next, excess slurrywas removed, and the substrate was then dried at 100° C. and heattreated (fired) for 1 hour at 500° C. A catalyst layer was formed on thesurface of the substrate in this way. An exhaust gas purifying catalystaccording to the present example was prepared in this way.

Example 2

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 191 g, the amount of the SiO₂ sol was changed to 49 g, theamount of the La₂O₃ sol was changed to 73 g, and the amount of the Cuion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) was changed to 208g.

Example 3

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 85 g, the amount of the SiO₂ sol was changed to 97 g, theamount of the La₂O₃ sol was changed to 146 g, and the amount of the Cuion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) was changed to 192g.

Example 4

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 9.4 g, the amount of the SiO₂ sol was changed to 146 g, theamount of the La₂O₃ sol was changed to 219 g, and the amount of the Cuion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) was changed to 149g.

Example 5

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 20 g, the amount of the SiO₂ sol was changed to 154 g, theamount of the La₂O₃ sol was changed to 231 g, and the amount of the Cuion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) was changed to 115g.

Example 6

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 242 g, the amount of the La₂O₃ sol was changed to 73 g, theamount of the Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10)was changed to 205 g, and the SiO₂ sol was not used.

Example 7

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=15).

Example 8

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 2, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=15).

Example 9

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 3, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=15).

Example 10

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 4, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=15).

Example 11

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 5, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=15).

Example 12

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 2, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(AEI, SiO₂/Al₂O₃ molar ratio=10).

Example 13

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 2, except that the Cu ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=10) was replaced with a Cu ion exchange zeolite(AFX, SiO₂/Al₂O₃ molar ratio=10).

Example 14

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 225 g, the amount of the SiO₂ sol was changed to 97 g, theamount of the Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10)was changed to 197 g, and the La₂O₃ sol was not used.

Example 15

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 1, except that the amount of pure water waschanged to 225 g, the amount of the SiO₂ sol was changed to 97 g, the Cuion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=10) was replaced with197 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=15), andthe La₂O₃ sol was not used.

Example 16

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 14, except that the Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=10) was replaced with 197 g of a Cu ionexchange zeolite (AEI, SiO₂/Al₂O₃ molar ratio=10).

Example 17

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 14, except that the Cu ion exchange zeolite(CHA, SiO₂/Al₂O₃ molar ratio=10) was replaced with 197 g of a Cu ionexchange zeolite (AFX, SiO₂/Al₂O₃ molar ratio=10).

The structures of the Cu ion-exchanged zeolites used, the SiO₂/Al₂O₃molar ratios and the molar ratios of La₂O₃ relative to SiO₂ (La₂O₃/SiO₂)contained in the Cu ion-exchanged zeolites for the exhaust gas purifyingcatalysts of these examples are summarized in Table 1.

(2) Element Mapping

A cross section of La₂O₃-adhered zeolite particles of Example 2 wasmeasured using a field emission type Electron Probe Micro Analyzer(FE-EPMA), and mapping analysis was carried out for each element. Theresults are shown in FIGS. 4 to 7. FIG. 4 shows a secondary electronimage, FIG. 5 shows a Si element mapping image, FIG. 6 shows an Alelement mapping image, and FIG. 7 shows a La element mapping image. Asshown in FIG. 5, the detected amount (concentration) of Si elementincreased towards the inner part of the zeolite particles. In addition,the detected amount (concentration) of Al element was approximately thesame at the surface and in the inner part of the zeolite particles, asshown in FIG. 6. As shown in FIG. 7, however, in the La₂O₃-adheredzeolite particles of Example 2, the detected amount (concentration) ofLa element present at the surface of the zeolite particles was higherthan the detected amount (concentration) of La element present in theinner part of the zeolite particles. That is, it was confirmed that Laelement was localized at surface parts of the zeolite particles.

(3) FT-IR Evaluation

The La₂O₃-adhered zeolite particles of Example 2 and the zeoliteparticles of Example 14 were measured using a Fourier transform infraredspectrophotometer (FT-IR), and it was confirmed that a silanol grouppeak was observed at a wavelength close to 3750 cm⁻¹. The results areshown in FIG. 8. As shown in FIG. 8, the La₂O₃-adhered zeolite particlesof Example 2 had a smaller silanol group peak than the zeolite particlesof Example 14. From these results, it was confirmed that the amount ofsilanol groups at the surface of zeolite particles is reduced byaddition of La₂O₃.

(4) Hydrothermal Durability Test

The hydrothermal durability of the exhaust gas purifying catalysts ofthe examples was evaluated. A hydrothermal durability test was carriedout by holding an exhaust gas purifying catalyst for 30 hours at 750° C.in a gas atmosphere containing 10% of H₂O. In addition, following thehydrothermal durability test, a model gas (NH₃=500 ppm, NO=500 ppm,O₂=10%, H₂O=5%, N₂=balance) was subjected to NOx purifying by beingpassed through the exhaust gas purifying catalyst at a temperature of200° C. The SV (space velocity) was 86,000 h⁻¹. In addition the NOxconcentration in the gas introduced into the catalyst and the NOxconcentration in the gas discharged from the catalyst were measured, andthe NOx purifying rate was calculated using the formula below. Theresults are shown in Table 1, FIGS. 9 and 10. FIG. 9 is a graph in whichNOx purifying rates are compared for Examples 1 to 6 and 14. FIG. 10 isa graph in which NOx purifying rates are compared for Examples 7 to 11and 15.

NOx purifying rate (%)=[(total NOx amount introduced intocatalyst)−(total NOx amount discharged from catalyst)]/(total NOx amountintroduced into catalyst)

TABLE 1 Zeolite NOx SiO₂/Al₂O₃ La₂O₃/SiO₂ purifying Structure ratiomolar ratio rate (%) Example 1 CHA 10 0.00113 80.66 Example 2 CHA 100.00395 82.58 Example 3 CHA 10 0.00789 81.88 Example 4 CHA 10 0.0135375.1 Example 5 CHA 10 0.01691 69.04 Example 6 CHA 10 0.00395 82.1Example 7 CHA 15 0.00109 75.64 Example 8 CHA 15 0.00383 79.78 Example 9CHA 15 0.00765 80.01 Example 10 CHA 15 0.01312 79.2 Example 11 CHA 150.0164 70.23 Example 12 AEI 10 0.00395 84.5 Example 13 AFX 10 0.0039583.1 Example 14 CHA 10 0 68.68 Example 15 CHA 15 0 67.72 Example 16 AEI10 0 75.05 Example 17 AFX 10 0 73.6

As shown in Table 1 and FIGS. 9 and 10, the catalysts of Examples 14 and15, to which La₂O₃ was not added, had NOx purifying rates of less than70% following the hydrothermal durability test and were poor in terms ofdurability. The reason for this is thought to be that in Examples 14 and15, the zeolites were readily attacked by water due to the presence ofsilanol groups. As a result, it is surmised that these zeolitesunderwent structural breakage and the NOx purifying rate following thehydrothermal durability test decreased. In addition, the samples ofExamples 5 and 11, in which the La₂O₃/SiO₂ molar ratio was 0.016 ormore, had NOx purifying rates of less than 70% following thehydrothermal durability test and were poor in terms of durability. It issurmised that the reason for this is that in Examples 5 and 11, La₂O₃excessively covered the surface of the zeolite particles, meaning thatgas permeability decreased and the NOx purifying rate of the catalyst asa whole decreased.

Conversely, the samples of Examples 1 to 4 and 6 to 10, in which theLa₂O₃/SiO₂ molar ratio was 0.001 to 0.014, had NOx purifying rates of70% or more following the hydrothermal durability test and maintained ahigh NOx purifying rate even after the durability test. From theseresults, it could be confirmed that by adding La₂O₃ to a zeolite suchthat the La₂O₃/SiO₂ molar ratio was 0.001 to 0.014, the hydrothermaldurability of the catalyst could be effectively improved without causinga decrease in the NOx purifying performance of the catalyst as a whole.

In addition, the samples of Examples 12 and 13, in which the La₂O₃/SiO₂molar ratio was 0.001 to 0.014 and an AEI or AFX zeolite was used,exhibited improved NOx purifying rates following the hydrothermaldurability test than Examples 16 and 17, in which La₂O₃ was not addedand an AEI or AFX zeolite was used. From these results, it could beconfirmed that the advantageous effect of the feature disclosed here,that is, the advantageous effect of improving the NOx purifying rate byadding La₂O₃ to a zeolite, could be achieved regardless of the type andstructure of the zeolite.

TEST EXAMPLE 2

In the present example, the following test was carried out in order toconfirm the effect of the average particle diameter D1 of the zeoliteparticles and the average particle diameter D2 of the rare earthelement-containing compound on purifying performance

Example 18

216 g of pure water, 48 g of a SiO₂ sol, 36 g of a La₂O₃ sol (La₂O₃content: 20%) and 200 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃molar ratio=13) were mixed and stirred for 15 minutes. A slurry ofLa₂O₃-adhered zeolite particles was prepared in this way. The La₂O₃ solwas added at a quantity whereby the molar ratio of La contained in theLa₂O₃ relative to Si contained in the zeolite (La₂O₃/SiO₂) was 0.00811in terms of oxides. In addition, the amount of water in the slurry wasadjusted so as to attain a slurry viscosity such that the slurry couldbe coated on a cordierite honeycomb substrate. The obtained slurry wascoated on a cordierite honeycomb substrate at a quantity whereby thecoating amount of the Cu ion exchange zeolite (the mass per 1 L ofsubstrate volume) was 180 g/L after firing. Next, excess slurry wasremoved, and the substrate was then dried at 100° C. and heat treated(fired) for 1 hour at 500° C. A catalyst layer was formed on the surfaceof the substrate in this way. Moreover, the La₂O₃ had an averageparticle diameter D2 of 150 nm, as measured using a dynamic lightscattering method, and the zeolite particles had an average particlediameter D1 of 0.4 μm, as measured by means of FE-SEM. An exhaust gaspurifying catalyst according to the present example was prepared in thisway.

Example 19

180 g of pure water, 48 g of a SiO₂ sol, 72 g of a La₂O₃ sol (La₂O₃content: 10%) and 200 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃molar ratio=13) were mixed and stirred for 15 minutes. A slurry ofLa₂O₃-adhered zeolite particles was prepared in this way. Moreover, theLa₂O₃ had an average particle diameter D2 of 30 nm, as measured using adynamic light scattering method, and the zeolite particles had anaverage particle diameter D1 of 0.4 μm, as measured by means of FE-SEM.Other than this, the exhaust gas purifying catalyst was prepared usingthe same procedure as that used in Example 18.

Example 20

233 g of pure water, 48 g of a SiO₂ sol, 3 g of La(NO₃)₃.6H₂O and 200 gof a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13) were mixedand stirred for 15 minutes. A slurry of La(NO₃)₃-adhered zeoliteparticles was prepared in this way. On the assumption that the La(NO₃)₃is converted into La₂O₃ during firing, the La(NO₃)₃.6H₂O was added at aquantity whereby the molar ratio of La contained in the La₂O₃ relativeto Si contained in the zeolite (La₂O₃/SiO₂) is 0.00116 in terms ofoxides. Moreover, the La(NO₃)₃.6H₂O had an average particle diameter D2of 1.0 nm, as measured using a dynamic light scattering method, and thezeolite particles had an average particle diameter D1 of 0.4 μm, asmeasured by means of FE-SEM. Other than this, the exhaust gas purifyingcatalyst was prepared using the same procedure as that used in Example18.

Example 21

233 g of pure water, 48 g of a SiO₂ sol, 10 g of La(NO₃)₃.6H₂O and 200 gof a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13) were mixedand stirred for 15 minutes. A slurry of La(NO₃)₃-adhered zeoliteparticles was prepared in this way. On the assumption that the La(NO₃)₃is converted into La₂O₃ during firing, the La(NO₃)₃.6H₂O was added at aquantity whereby the molar ratio of La contained in the La₂O₃ relativeto Si contained in the zeolite (La₂O₃/SiO₂) is 0.00406 in terms ofoxides. Moreover, the La(NO₃)₃.6H₂O had an average particle diameter D2of 1.0 nm, as measured using a dynamic light scattering method, and thezeolite particles had an average particle diameter D1 of 0.4 pm, asmeasured by means of FE-SEM. Other than this, the exhaust gas purifyingcatalyst was prepared using the same procedure as that used in Example18.

In the present example, aggregation of La-containing particles occurredduring firing because the added quantity of La(NO₃)₃.6H₂O was higherthan in Example 22. As a result, the average particle diameter D2, asmeasured by means of FE-SEM, of the La-containing particles adhered tothe zeolite particles was 1.0 μm.

Example 22

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 18, except that the La₂O₃ sol was not added.

Example 23

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 18, except that a Fe ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=13, average particle diameter=0.4 μm) was usedinstead of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13,average particle diameter=0.4 μm).

Example 24

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 19, except that a Fe ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=13, average particle diameter=0.4 μm) was usedinstead of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13,average particle diameter=0.4 μm).

Example 25

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 20, except that a Fe ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=13, average particle diameter=0.4 μm) was usedinstead of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13,average particle diameter=0.4 μm).

Example 26

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 21, except that a Fe ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=13, average particle diameter=0.4 μm) was usedinstead of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13,average particle diameter=0.4 μm).

In the present example, the average particle diameter D2, as measured bymeans of FE-SEM, of the La-containing particles adhered to the zeoliteparticles was 1.0 μm because the added quantity of La(NO₃)₃.6H₂O washigher than in Example 25 and aggregation of La-containing particlesoccurred during firing.

Example 27

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 22, except that a Fe ion exchange zeolite (CHA,SiO₂/Al₂O₃ molar ratio=13, average particle diameter=0.4 μm) was usedinstead of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃ molar ratio=13,average particle diameter=0.4 μm).

The types of rare earth element-containing compounds used, thestructures of the ion-exchanged zeolites used, the ion exchange cations,the SiO₂/Al₂O₃ molar ratios, the molar ratios of La₂O₃ relative to SiO₂(La₂O₃/SiO₂) contained in the ion-exchanged zeolites and the ratios(D2/D1) of the average particle diameter D2 of the rare earthelement-containing compound relative to the average particle diameter D1of the zeolite particles for the exhaust gas purifying catalysts of theexamples are summarized in Table 2.

The hydrothermal durability of the exhaust gas purifying catalysts ofthe examples was evaluated. A hydrothermal durability test was carriedout by holding an exhaust gas purifying catalyst for 30 hours at 750° C.in a gas atmosphere containing 10% of H₂O. In addition, following thehydrothermal durability test, a model gas (NH₃=500 ppm, NO=500 ppm,O₂=10%, H₂O=5%, Na=balance) was subjected to NOx purifying by beingpassed through the exhaust gas purifying catalyst at a prescribedtemperature (200° C. in Examples 18 to 22, and 400° C. in Examples 23 to27). The SV (space velocity) was 86,000 h⁻¹. In addition the NOxconcentration in the gas introduced into the catalyst and the NOxconcentration in the gas discharged from the catalyst were measured, andthe NOx purifying rate was calculated using the formula below. Theresults are shown in Table 2, FIGS. 11 and 12. FIG. 11 is a graph inwhich NOx purifying rates are compared for Examples 18 to 22. FIG. 12 isa graph in which NOx purifying rates are compared for Examples 23 to 27.

NOx purifying rate (%)=[(total NOx amount introduced intocatalyst)−(total NOx amount discharged from catalyst)]/(total NOx amountintroduced into catalyst)

TABLE 2 Table 2 Rare earth Zeolite NOx element- Ion SiO₂/ La₂O₃/purifying containing exchange Al₂O₃ SiO₂ rate compound Structure cationmolar ratio molar ratio D2/D1 (%) Example La₂O₃ CHA Cu 13 0.00811 0.37579.8 18 Example La₂O₃ CHA Cu 13 0.00811 0.075 80.8 19 Example La(NO₃)₃ ·CHA Cu 13 0.00116 0.0025 71.5 20 H₂O Example La(NO₃)₃ · CHA Cu 130.00406 2.5 70.3 21 H₂O Example — CHA Cu 13 — — 68.3 22 Example La₂O₃CHA Fe 13 0.00811 0.375 82.5 23 Example La₂O₃ CHA Fe 13 0.00811 0.07583.1 24 Example La(NO₃)₃ · CHA Fe 13 0.00116 0.0025 74.7 25 H₂O ExampleLa(NO₃)₃ · CHA Fe 13 0.00406 2.5 74.0 26 H₂O Example — CHA Fe 13 — —72.2 27

As shown in Table 2 and FIGS. 11 and 12, the catalysts of Examples 18 to21, in which a La-containing compound was added at a quantity wherebythe La₂O₃/SiO₂ molar ratio was 0.001 to 0.014, exhibited better NOxpurifying rates following the durability test than Example 22. Inparticular, the catalysts of Examples 18 and 19, in which the ratio(D2/D1) of the average particle diameter D2 of the La-containingcompound relative to the average particle diameter D1 of the zeoliteparticles was such that 0.005<(D2/D1)<0.5, exhibited even better NOxpurifying rate results following the durability test than Examples 20and 21. In addition, the catalysts of Examples 23 to 26, in which aLa-containing compound was added at a quantity whereby the La₂O₃/SiO₂molar ratio was 0.001 to 0.014, exhibited a better NOx purifying ratefollowing the durability test than Example 27. In particular, thecatalysts of Examples 23 and 24, in which the ratio (D2/D1) of theaverage particle diameter D2 of the La-containing compound relative tothe average particle diameter D1 of the zeolite particles was such that0.005<(D2/D1)<0.5, exhibited even better NOx purifying rate resultsfollowing the durability test than Examples 25 and 26.

TEST EXAMPLE 3

In the present example, the following test was carried out in order toconfirm the effect of the added quantity of the rare earthelement-containing compound on purifying performance in zeolitescontaining a large quantity of Al (SiO₂/Al₂O₃=7.5).

Example 28

175 g of pure water, 66 g of a SiO₂ sol, 26 g of a La₂O₃ sol (La₂O₃content: 10%) and 175 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃molar ratio=7.5) were mixed and stirred for 15 minutes. A slurry ofLa₂O₃-adhered Cu zeolite particles was prepared in this way. The La₂O₃sol was added at a quantity whereby the molar ratio of La contained inthe La₂O₃ relative to Si contained in the zeolite (La₂O₃/SiO₂) was0.00368 in terms of oxides. In addition, the amount of water in theslurry was adjusted so as to attain a slurry viscosity such that theslurry could be coated on a cordierite honeycomb substrate. The obtainedslurry was coated on a cordierite honeycomb substrate at a quantitywhereby the coating amount of the Cu ion exchange zeolite (the mass per1 L of substrate volume) was 180 g/L after firing. Next, excess slurrywas removed, and the substrate was then dried at 100° C. and heattreated (fired) for 1 hour at 500° C. A catalyst layer was formed on thesurface of the substrate in this way. Moreover, the La₂O3 had an averageparticle diameter D2 of 40 nm, as measured using a dynamic lightscattering method, and the zeolite particles had an average particlediameter D1 of 0.37 μm, as measured by means of FE-SEM. An exhaust gaspurifying catalyst according to the present example was prepared in thisway.

Example 29

222 g of pure water, 42 g of a SiO₂ sol, 61 g of a La₂O₃ sol (La₂O₃content: 10%) and 175 g of a Cu ion exchange zeolite (CHA, SiO₂/Al₂O₃molar ratio=7.5) were mixed and stirred for 15 minutes. A slurry ofLa₂O₃-adhered Cu zeolite particles was prepared in this way. The La₂O₃sol was added at a quantity whereby the molar ratio of La contained inthe La₂O₃ relative to Si contained in the zeolite (La₂O₃/SiO₂) was0.00858 in terms of oxides. Other than this, the exhaust gas purifyingcatalyst was prepared using the same procedure as that used in Example28.

Example 30

134 g of pure water, 70 g of a SiO₂ sol, 102 g of a La₂O₃ sol (La₂O₃content: 10%) and 180 g of a Cu ion exchange zeolite (CHA structure,SiO₂/Al₂O₃ molar ratio=7.5) were mixed and stirred for 15 minutes. Aslurry of La₂O₃-adhered Cu zeolite particles was prepared in this way.The La₂O₃ sol was added at a quantity whereby the molar ratio of Lacontained in the La₂O₃ relative to Si contained in the zeolite(La₂O₃/SiO₂) was 0.01384 in terms of oxides. Other than this, theexhaust gas purifying catalyst was prepared using the same procedure asthat used in Example 28.

Example 31

An exhaust gas purifying catalyst was prepared using the same procedureas that used in Example 28, except that the La₂O3 sol was not added.

The types of rare earth element-containing compounds used, thestructures of the ion-exchanged zeolites used, the ion exchange cations,the SiO₂/Al₂O₃ molar ratios, the molar ratios of La₂O₃ relative to SiO₂(La₂O₃/SiO₂) contained in the ion-exchanged zeolites and the ratios(D2/D1) of the average particle diameter D2 of the rare earthelement-containing compound relative to the average particle diameter D1of the zeolite particles for the exhaust gas purifying catalysts of theexamples are summarized in Table 3.

The hydrothermal durability of the exhaust gas purifying catalysts ofthese examples was evaluated under the same conditions as those used inTest Example 1. In addition the NOx concentration in the gas introducedinto the catalyst and the NOx concentration in the gas discharged fromthe catalyst were measured, and the NOx purifying rate was calculatedusing the formula below. The results are shown in Table 3 and FIG. 13.FIG. 13 is a graph in which NOx purifying rates are compared forExamples 28 to 31.

NOx purifying rate (%)=[(total NOx amount introduced intocatalyst)−(total NOx amount discharged from catalyst)]/(total NOx amountintroduced into catalyst)

TABLE 3 Table 3 Rare earth Zeolite NOx element- Ion SiO₂/ La₂O₃/purifying containing exchange Al₂O₃ SiO₂ rate compound Structure cationmolar ratio molar ratio D2/D1 (%) Example La₂O₃ CHA Cu 7.5 0.00368 0.10888.2 28 Example La₂O₃ CHA Cu 7.5 0.00858 0.108 91.5 29 Example La₂O₃ CHACu 7.5 0.01384 0.108 84.5 30 Example — CHA Cu 7.5 — 0.108 73.6 31

As shown in Table 3 and FIG. 13, the catalysts of Examples 28 to 30, inwhich a La-containing compound was added at a quantity whereby theLa₂O₃/SiO₂ molar ratio was 0.001 to 0.014, exhibited better NOxpurifying rates following the durability test than Example 31. Of these,the catalysts of Examples 28 to 29, in which a La-containing compoundwas added at a quantity whereby the La₂O₃/SiO₂ molar ratio was 0.0035 to0.009, exhibited extremely good NOx purifying rates following thedurability test. From these results, it could be confirmed that theadvantageous effect of the feature disclosed here, that is, theadvantageous effect of improving the NOx purifying rate by adding La₂O₃to a zeolite, could be achieved regardless of the type of the zeolite.

In addition, from a comparison between Examples 1 to 4 and 6 to 10 inTest Example 1 and Examples 28 to 30 in Test Example 3, it can be seenthat Examples 28 to 30, in which the SiO₂/Al₂O₃ molar ratio in thezeolite was 7 to 10, achieved better results in terms of NOx purifyingrate following the durability test. The reason for this is thought to bebecause the number of active sites in the catalyst increased because thesupported amount of Cu was higher and because the adsorbed amount ofammonia was higher.

Specific examples of the present invention have been explained in detailabove, but these are merely examples, and do not limit the scope of theinvention. The features disclosed in the claims also encompass modesobtained by variously modifying or altering the specific examples shownabove.

REFERENCE SIGNS LIST 10 Substrate

20 Catalyst layer22 Zeolite particle24 Rare earth element-containing compound

26 Binder

100 Exhaust gas purifying catalyst

1. A catalyst body which is used in an exhaust gas purifying catalyst,the catalyst body comprising: zeolite particles; a metal supported onthe zeolite particles; and a rare earth element-containing compounddisposed on a surface of the zeolite particles, wherein the rare earthelement-containing compound contains lanthanum (La) as a rare earthelement, and an amount of the rare earth element-containing compound issuch an amount that a molar ratio of the rare earth element relative toSi contained in the zeolite particles is 0.001 to 0.014 in terms ofoxides.
 2. The catalyst body according to claim 1, wherein arelationship between an average particle diameter D1 of the zeoliteparticles and an average particle diameter D2 of the rare earthelement-containing compound satisfies the following formula:0.005<(D2/D1)<0.5.
 3. The catalyst body according to claim 1, wherein anaverage particle diameter D2 of the rare earth element-containingcompound is 100 nm or less.
 4. The catalyst body according to claim 1,wherein when an amount of the rare earth element at a cross section of azeolite particle is measured using an Electron Probe Micro Analyzer(EPMA), the amount of the rare earth element present at the surface ofthe zeolite particle is greater than the amount of the rare earthelement present in the inner part of the zeolite particle.
 5. Thecatalyst body according to claim 1, wherein the rare earthelement-containing compound contains at least one of lanthanum oxide andlanthanum hydroxide.
 6. The catalyst body according to claim 1, whereinthe metal supported on the zeolite particles is Cu or Fe.
 7. Thecatalyst body according to claim 1, wherein a molar ratio of Si and Alin terms of oxides (SiO₂/Al₂O₃) in the zeolite particles is 5 to
 20. 8.The catalyst body according to claim 1, wherein the zeolite particlescontain at least one type of zeolite selected from among thoserepresented by structure codes CHA, AFX, AEI, LTA and BEA defined by theInternational Zeolite Association (IZA).
 9. The catalyst body accordingto claim 2, wherein the average particle diameter D2 of the rare earthelement-containing compound is 100 nm or less.